1. Field of the Invention
N,N, disubstituted p-phenylenediamine color developing agents, their incorporation as a principal functional component of a liquid packaged developer concentrate, their incorporation in color developing working solutions (baths) and their use in the color development of colored film.
2. Description of the Prior Art
The formation of colored negative or positive images from exposed sensitized silver halide color emulsions is a demanding process which requires a high degree of purity in the color developing solutions. The chemistry and physics of the process are described by J. R. Thirtle and D. M. Zwick in Kirk-Othmer "Encyclopedia of Chemical Technology", Volume 5, pages 812-845 (2nd edition). The process is applicable to the developing of photographic color negatives, photographic color positives, photographic color plates and colored cinematographic films. The developers currently and for many years past used in the trade to process such emulsions contain as the color developing agent N,N disubstituted p-phenylenediamines. In the course of color development, the color developing agent reacts with phenolic or active methylene couplers that are incorporated in sundry layers of the emulsion on a transparent base or paper and forms non-diffusing dyes that constitute the final image.
Such color developing agents, when in contact with exposed silver halide, distributed in three separate emulsion layers, undergo oxidation to quinone diimines which, in turn, condense with the three different color couplers, each of which is present in a different emulsion layer. The amount of quinone diimine formed is directly proportional to the amount of silver halide that has been exposed and, therefore, determines the amount and, hence, the intensity, of dye that is formed. This process is known as oxidative coupling. The combination of the layers of yellow, cyan and magenta dyes in amounts determined by the exposed silver halide can be made to reproduce with a high degree of faithfulness the colors of the photographed subject.
Any degradation of the color developing agent affects the amount of dye formed and the degradation products may adversely affect the color of the dye formed and also the degraded color agent may diffuse throughout a given layer to develop color in non-exposed areas. It will be appreciated by those skilled in the art that N,N, disubstituted p-phenylenediamines as color developing agents are characterized by their ability to condense with color couplers in the immediate vicinity of exposed silver halide grains and not to diffuse, whereas degradation products may not be characterized by such non-diffusibility and, hence, can result in non-selective dye formation.
From a practical point of view, the choice of developing agents is limited because of the characteristics demanded of the dyes, among others, optimum spectral characteristics and resistance to fading, bearing in mind that the most widely distributed color films are made by and under the control of a single manufacturer, so that the color developing agents are specifically tailored to the existing color couplers present in such emulsions in order to form the correct dyes, or because of characteristics demanded of the color developing agents such as non-allergenicity.
One particular N,N, disubstituted p-phenylenediamine color developing agent has gained wide acceptance in the trade and is the color developing agent recommended by the single manufacturer for color developing its color film. This is 4-amino-N-ethyl, N-beta methanesulfonamidoethyl-m-toluidene, hereinafter denominated as AEMT. This compound also is known as 4-amino, 3-methyl,N-ethyl,N-beta methanesulfonamidoethyl aniline. AEMT has the following structural formula: ##STR1## As a sesqui-sulfate monohydrate of the aforesaid free base, the color developing agent is marketed under the designation CD-3 and is used in a majority of color photographic processes developed by Eastman Kodak Company. Other N,N, disubstituted p-phenylenediamine color developing agents have been proposed, but none has found as wide an acceptance as CD-3 and, in the present market, CD-3 is the color developing agent of choice despite the fact that other N,N, disubstituted p-phenylenediamines are well known for the sample purpose. Typical of other N,N, disubstituted p-phenylenediamine color developing agents that form non-diffusing dyes with phenolic and active methylene couplers are N,N diethyl-p-phenylenediamine monohydrochloride [known in the trade as CD-1]; 4-amino-3-methyl-N-ethyl-N-beta hydroxyethyl aniline sulfate [the free base, as distinguished from the sulfate salt, is known as AMEHEA, the sulfate salt is known in the trade as CD-4]; 2-amino-5-diethyl-amino-toluene [known as ADAT, the hydrochloride salt is known in the trade as CD-2]; and similar N,N, disubstituted p-phenylenediamine color developing agents such, for example, as those disclosed in U.S. Pat. Nos. 2,552,241; 2,566,271 and 3,658,525, and in an article entitled "CHEMICAL CONSTITUTION, ELECTROMECHANICAL, PHOTOGRAPHIC AND ALLERGENIC PROPERTIES OF p-AMINO-N-DIALKYL-ANILINES" (sic) published in the Journal of the American Chemical Society, Volume 73, Pages 3100-3125 (1951).
As mentioned above, the color developing agent of primary commercial interest is, and for some time past has been, the sesqui-sulfate monohydrate of AEMT. This salt is itself a color developing agent. However, it does not constitute, as such, a color developer, that is to say, a working bath for the color development of a color film, but is a necessary ingredient of a color developer. In some commercial color processes it is the practice to segregate different constituents of a working color developer bath into separate components (compositions), each of which is concentrated as far as is practical, and each of which is liquid to facilitate mixing and dilution with water (upon occasion some of the components may be dry), and each of which is compartmented, that is to say packaged in a receptacle separate from other packaged components, all of the packaged components of a developer usually being contained in a single box. Any given component need not be, and usually is not, composed of a single ingredient such, for example, as a color developing agent. It is common, for example, to include a liquid solvent which may be water and/or an organic solvent and/or a solubilizing agent and/or a mixture of different liquids as an ingredient of each component. The organic solvent and/or solubilizing agent, if present, may have as their sole function that of a carrier or they also may have a photographic function such, for example, as boosting the dye formed when the color developing agent reacts with a color coupler. Additionally, there are various adjuvants which are used in a working color developer bath, examples of which are pH buffering systems, preservatives, sequestrants, anti-foggants, enhancers, and the like. Some of these require different solvent systems and, for such reason, are separately compartmented. Others tend to be unstable when kept for periods of time in the same compartment with different ones of the compounds used. Others have a tendency to react with different ones of the compounds used. Others require the presence of co-solvents, etc. Hence, separate compartmenting of different components is an accepted practice in the art. The principles involving separate compartmentation and concentrating and allocation of different compounds to different compartments, as well as the concept of mixing the different compartmented compounds and diluting the same with water at the time of use, are well known.
In connection with the color developing agent of principal interest, CD-3, the color developing component which includes the color developing agent is, in its concentrated form, dissolved in water and pre-packaged; subsequently, the concentrated color developing component is, as indicated above, added to other concentrated components and diluted with water to form a working bath. It has been found that when CD-3 is used, a protective agent must be employed in the color developing component (concentrate) containing the same to prevent immediate initiation of the decomposition of the color developing agent by aerial oxidation. Conventionally, this has been done by adding an alkaline bisulfite or sulfite, usually an alkaline metal bisulfite or sulfite, to the color developing component as an element thereof. The term "alkaline" includes such moieties as alkali metals, quaternary ammoniums, amines, etc.
A color developing concentrated component so composed and stored in a fully closed container undergoes slow decomposition and this decomposition is accelerated if the concentrate has its temperature raised by storage in a warm or even hot environment. Such slow decomposition is inevitable in commercial practice because, after the concentrate is prepared at a factory, it is conventional to store the same at a warehouse near the point of manufacture, then to ship it to distribution points where, again, it is stored until withdrawn upon order. Thereafter, it is trucked to a retail outlet where it is put in a back room for a considerable time, after which it is moved onto retail shelves where it awaits purchase by members of the public or by photographic processors. The prevailing temperature at each of its points of repose obviously is beyond the control of the manufacturer and, because of extremely wide geographic distribution, the color developing concentrate inevitably is subjected to warm and hot environments so that decomposition, if it is to take place, must occur. The exact nature of such decomposition is not fully understood by the present inventors, but it is theorized that it involves reactions between the sulfur/oxygen moieties present in the concentrate which, in turn, may in part turn upon the proportions between the color developing agent and the preservative. In any event, the photographic consequences of the decomposition are easily measured and it is these consequences which limit the shelf life of such concentrates, to wit, of a CD-3/sulfite concentrate.
When a container having such a CD-3/sulfite concentrate is opened and exposed to air, two disadvantageous reactions occur in addition to the previous in-package slow decomposition. One of these further reactions is that the liquid concentrate immediately evolves sulfur dioxide gas and, at the same time, the sulfite (or bisulfite) in the solution starts to decompose continuously and rapidly at a rate much faster than decomposition took place in the closed container. This decomposition proceeds until the sulfite or bisulfite is fully depleted. The second reaction is that exposure to air causes oxidation of the color developing agent. Initially, decomposition is comparatively slow due to the protective effect of the sulfite or bisulfite, but as this anti-oxidant depletes, the oxidation of the developing agent accelerates quite quickly until the decomposition proceeds very rapidly when the bisulfite/sulfite in the container diminishes to zero.
The decomposition of the color developing agent leads to the production of false colors, non-selective dye formation and fog in any color emulsion which is developed therewith. The evolution of sulfur dioxide gas from the concentrate when the container is opened is considered highly objectionable due to both the sharp pungent odor of the gas and to its lacrimating action.
The decomposition of the color developing agent and of the preservative could be somewhat reduced if the color developing component were not highly concentrated, but manufacturers and users find it advantageous, for reasons mentioned earlier, to make and use a color developing component which is as concentrated as it is reasonably possible to make. The concentration is limited practically by factors such as increase in viscosity to the point that the concentrate cannot be poured out of its container with ease, or there is too much of a tendency to retain some of the concentrate as a thick film on the walls of the container, or the viscosity becomes so high that dilution with water to form the working bath becomes overly difficult. Increase in concentration is also limited by the decomposition just discussed.